1. Field of the Invention
The present invention relates to an optical guide waveguide element, a manufacturing method for optical waveguide elements, an optical deflection element and an optical switching element, and more particularly to an optical waveguide element capable of being coupled with an optical fiber at high coupling efficiency and a manufacturing method for such optical waveguide elements, an optical deflection element and an optical switching element to which the optical waveguide element according to the invention is applied.
2. Description of the Related Art
Conventionally, glass such as silica, oxide ferroelectrics and electro-optical materials such as LiNbO3, magneto-optical materials such as Y3Ga5O12, polymers such as PMMA, and GaAs-based chemical compound semiconductors are used for planar optical waveguides. Among those materials, oxide ferroelectrics are known to manifest particularly satisfactory acousto-optical and electro-optical effects, but most of the actually produced acousto-optical elements and electro-optical elements use LiNbO3.
There are a wide variety of oxide ferroelectrics including LiNbO3, BaTiO3, PbTiO3, Pb1-xLax(ZryTi1−y)1-x/4O3 (which may be PZT, PLT or PLZT depending on the values of x and y), Pb(Mg1/3Nb2/3)O3, KNbO3, LiTaO3, SrxBa1-xNb2O6, PbxBa1-xNb2O3, Bi4Ti3O12, Pb2KNb5O13, and K3Li2Nb5O15, and many of them are superior in characteristics to LiNbO3. Especially, Pb1-xLax(ZryT1−y)1-x/4O3 is known as a material having a much higher electro-optical coefficient than LiNbO3. The electro-optical coefficient of LiNbO3 monocrystals is 30.9 pm/V whereas that of PLZT (8/65/35: x=8%, y=65%, 1−y=35%) ceramic is as high as 612 pm/V.
The reason why most of the elements actually produced LiNbO3 or LiTaO3, in spite of the availability of many ferroelectrics with better characteristics than LiNbO3, is that monocrystal growth technology and waveguide formation technology by Ti diffusion to wafers or proton exchange are well established for LiNbO3 and LiTaO3, while thin films need to be by epitaxial for other materials than LiNbO3 and LiTaO3, and conventional vapor-phase cannot provide thin film optical waveguides of high enough quality for practical use.
The present inventors, with a view to solving this problem, proposed a solid phase epitaxial growth technique capable of providing thin film optical waveguides of high enough quality for practical use (Japanese Published Unexamined Patent Application No. Hei 7-78508), but an epitaxially grown thin film optical guide often has to be thinner than the mode field diameter of the optical fiber on account of the requirement for singleness of the mode or that to reduce the drive voltage, inviting an increased loss in coupling with the optical fiber.
Previously, regarding semiconductor optical waveguides and silica-based waveguides, techniques to provide a flared optical waveguide in the position of connection to the optical fiber to reduce the coupling loss between the optical waveguide and the optical fiber were proposed in Japanese Published Unexamined Patent Application No. Hei 9-61652 and Japanese Published Unexamined Patent Application No. Hei 5-182948 among others.
However, there is no technique available for producing a fine pattern in the epitaxially grown oxide thin film optical waveguide, making it difficult to fabricate the optical waveguide in a flared shape. For LiNbO3 monocrystalline wafers for example, a fabrication method for three-dimensional optical waveguides to which Ti diffusion and proton exchange are described by Nishibara, Haruna and Suhara in a publication on optical integrated circuits by Ohmsha (1993) pp. 195-230, but no method for other elements or exchanging ions is known for other materials, especially Pb1-xLax(ZryTi1−y)1-x/4O3. For silica-based optical waveguides and the like, a method for producing channel optical waveguides by reactive ion etching is disclosed by Kawachi in NTT R&D, 43 (1994)1273 and elsewhere, but it is difficult to accomplish selectively etch a monocrystalline epitaxial ferroelectric thin film optical waveguide without roughing its surface, which would invite scattering loss or damaging the substrate, which is an oxide of the same kind as the thin film optical waveguide. For this reason, no report is found on the successful fabrication of any relatively loss-free channel optical waveguide into an epitaxial ferroelectric thin film optical waveguide. Furthermore, there is another problem that merely flaring an epitaxially grown oxide thin film optical waveguide can hardly prevent the waveguide mode from becoming multi-mode.